The present invention relates to an image forming apparatus having a plurality of imaging units.
In an electrophotographic image forming apparatus, an electrostatic latent image is formed on the surface of a uniformly charged photoconductor by exposure, and the electrostatic latent image is developed by a developing device to make a toner image, which is transferred to recording media such as recording paper or to intermediate transfer bodies such as intermediate transfer belts. The toner which has remained on the photoconductor after the transfer becomes waste toner if it is scraped and recovered by a cleaner, thus increasing a toner waste. Consequently, a so-called cleanerless process is known in which the residual toner on the photoconductor is adsorbed by a developing roller of the developing to be collected into the developing device so that the toner is provided for recycle.
However, if the cleanerless process is adopted in a tandem-type color image forming apparatus composed of a plurality of imaging units, each of which includes a photoconductor and a developing device and which is parallely placed along an intermediate transfer belt, some toner in the toner image, which was formed with the imaging unit placed in the upstream with respect to the moving direction of the intermediate transfer belt and which was transferred onto the intermediate transfer belt, is reversely transferred onto the photoconductor of an imaging unit located in the downstream so that toner of a different color is collected to the developing device of the imaging unit. This leads to color mixture and ends up causing the adverse influence on the color shade of a final color image.
Accordingly, in order to prevent generation of the color mixture, in JP 2000-181169 A, the toner is not collected in the second and further imaging units from the upstream.
in JP 2001-188394 A, when the image ratio of a toner image formed with an imaging unit on the downstream side is smaller than the image ratio of a toner image formed with an imaging unit on the upstream side, a toner image, which will not be transferred to the recording medium in the end, is formed in the imaging unit on the downstream side and is transferred to the intermediate transfer belt so as to be collected by a cleaner. This allows forcible consumption of the toner with color mixture and allows supply of new toner.
As for JP 2001-188394 A, the color mixture is a phenomenon in which the color mixture rate gradually increases as toner of different colors is mixed inside the developing device. Therefore, high image ratio of an imaging unit on the upstream side does not immediately cause any significant change in the color shade of the toner, and even if the image ratio of the imaging unit on the downstream side is below the image ratio of the imaging unit on the upstream side, color mixture such as causing change of color shade, which is unacceptable in formed images, may still occur after repeated operations.